Field of the Invention
The invention relates to a process and a device for producing components and semi-finished products from synthetic graphite or ceramic granules, in particular for producing graphite tubes or a component or semi-finished product made of synthetic graphite or ceramic granules.
Graphite has very good electrical and thermal conductivity and has a very high level of chemical and thermal resistance. For this reason the material graphite is used, in the form of tubes for example, as a semi-finished product for chemical apparatuses. Tubular heat exchangers made from bundled graphite tubes therefore are known for example.
As graphite is a ceramic material, one of the minor advantageous characteristics of this material is a relatively low resistance to impulsive and dynamic loads, in particular, in order to improve the dynamic resistance of graphite tubes, it was proposed in accordance with DE 31 16 309 C2 to cover the tubes with carbon fibres, the connection with interlocking fit between the tube and the bundles of fibres being produced by means of a curable resin, in a manner similar to a laminate. This process is, however, relatively complex and therefore cost intensive.
In accordance with a known process for producing components and semi-finished products from graphite, petroleum coke or graphite granules, for example, are used, as raw materials which are comminuted and screened in a first step. Subsequently, the bulk starting material which has been comminuted is mixed with a binding agent to form a viscous material to be mixed. For the shaping of the material to be mixed which is obtained in such a manner and is ready for pressing, for example to form a tube, this is fed into the supply chamber of an extrusion press and then compressed by means of a plunger piston which can be moved longitudinally in a press housing in the direction of an outlet opening and is pressed out as a virtually continuous strand, for example in the form of a tube. A screw-type extruder which is coaxial to the press housing can also assume the pressing out of the material instead of a plunger piston.
As a result of the forward movement of the plunger piston and the relatively good flowability of the material to be mixed, the predominantly elongate-shaped coke or graphite particles are aligned from the start in the case of extrusion pressing or extrusion so as to be parallel to the direction of pressing in the flow field pointing towards the outlet opening and remain “frozen” in this position in the subsequent production stages. As the main direction of thermal conduction runs primarily along the alignment of the material particles, the material properties of components pressed in such a way consequently have strong anisotropies, i.e. whilst the thermal conductivity of an extrusion pressed heat exchanger graphite tube has high values owing to the predominant particle alignment parallel to the direction of pressing in axial longitudinal direction, it is disadvantageously low, however, in the radial direction which is important for heat transference in heat exchanger tubes. On the other hand, the alignment of the particles in the axial longitudinal direction has a favourable effect on the bending and tensile strength of the graphite components which is particularly advantageous in heat exchange tubes.
Conventional extrusion presses have a plunger piston which can be moved longitudinally inside a press housing, which plunger piston delimits a supply chamber which discharges into an outlet opening of a shaping mouthpiece of the press housing narrowing in the shape of a funnel in the direction of pressing, which funnel can be filled with material to be pressed and can be comminuted by its pressing movement. Once the material to be pressed has been pressed out of the outlet opening, the plunger piston must be withdrawn completely from the press housing so the supply chamber can be refilled with material to be pressed. It goes without saying that the refilling process is complex and therefore the piston stroke and the supply chamber of such extrusion presses must be designed as large as possible for reasons of economy, in order to avoid frequent refilling. Accordingly, the pressed part located in the supply chamber is relatively long, whereby the forward movement forces required to compress and press out the pressed part from the constricted outlet opening are high. In order to limit the pressing forces, the bulk material is accordingly free-flowing. Upon pressing out, a flow field is produced inside the supply chamber, in which flow field the graphite particles tend to align themselves parallel to the direction of pressing which, in tubes, in turn results in the already described undesired anisotropy of the graphite material.